I don't think there's any single one "best way" to explain status effects. It depends on what your RPG's battle system is
about, who your target audience is, how hard it might be for players to observe and keep track of all the information for each status effect and if it even matters for the player to be successful, under what circumstances the player is going encounter each status effect for the first time, etc.
I think creativity often happens when you relate part of something to a greater whole, such as some other design aspect of your game. So for example, when is the first time the player is going to encounter the Pestilence status effect? If it were during a story boss battle with a mad scientist, for example, then perhaps you could have a NPC or party member who has a history with the mad scientist (former friend, former lover, former lab experiment, rival scientist?) comment that he's notorious for inflicting his guinea pigs with a viral "pestilence" that drains one's life over time, and weakens their strength and defenses, to see if they can survive it. In this way the player is provided with info they need when they need it, and it's also integrated with storytelling and characterization.
However, players might not remember that some NPC told them once upon a time that "Pestilence" not only drains HP but lowers both ATK and DEF. Players might wonder whether "Pestilence" belongs to the Poison/Venom category and if an Antidote will cure it (it's unclear to me from your post, and it could go either way). They might also forget that "Fear" lowers ATK but raises DEF (it makes sense, but other interpretations also make sense, e.g., in Super Mario RPG the Fear status effect lowers both ATK and DEF). Rules like these are more or less arbitrary, and hence harder to keep straight, especially as the number of status effects increase. This is where it's helpful to provide an in-battle status menu or manual/compendium that the player can access anywhere. If your game's battles are fairly easy (like Pokemon story mode or something) then a status menu or manual thingie might not be necessary since the player can win without the information. But if your game's battles are fairly challenging such that knowing what the status effects do and how to cure them is really important, then players might get frustrated if they can't look up information they need or if they need to jump through tedious hoops to find it (like backtracking, trial and error).
But I want to nitpick the suggestion of using "classic RPG states" as the most common ones. Think about these states and whether they present any kind of interesting decisions to the player:
- Sleep: Very interesting tradeoff between damage and disable when it's inflicted on an enemy, but less so when it's inflicted on a character and the AI is attacking randomly
- Poison: Interesting decision between trying to finish the battle quickly, trying to heal through it, or using the turn to cure the afflicted character
- Paralyzed: No interesting decisions to be made; the battler is disabled (and good as dead) and there's nothing that can be done about it except use the appropriate item
- Sealed / Silenced: Generally just means attackers keep attacking, and mages just hit Guard each turn
- Confused: Strategic decisions vary widely based on the situation, but in most games it just means hoping for a benevolent RNG
- Petrify: No interesting decisions to be made; the battler is disabled (and good as dead) and there's nothing that can be done about it except use the appropriate item
As you can see, less than half actually add anything interesting to the battle, and are really not worth the potential frustration they cause for the player. I think in general "positive" statuses tend to make for a better experience, but even for negative statuses I think it's very important to make sure they add, rather than take away, from the player's set of interesting possibilities.
That's an interesting observation, that a lot of common status effects are merely irritants when they force the player to make one choice (using the appropriate item or spell to cure the status effect).
To be fair, there usually are more player decisions involved even with the conventional status effects of an old-school jRPG, assuming the game is reasonably well done like a Dragon Quest or Final Fantasy. This is especially the case when:
1) Curative items or spells are costly or rare (Do I need to cure this status effect immediately, and risk having being reapplied next battle or even next turn, or can I let it ride for a bit? What unit/job classes should I bring into a dungeon to deal with the status effects I might encounter? How long will their MP hold out before I have to fall back on my limited stock of items? What's the bare minimum of curative items I need to buy to get me through this dungeon?)
2) Multiple status effects are applied on multiple characters (How should I prioritize curing which status effects and on which party members?)
3) Accessories that resist certain status effects are costly or rare (Which party member should I equip with this accessory? If I can purchase more of them is it worth the price? Is it worth placing in my limited accessory slot or would another accessory with another effect be better?)
The problem is that many RPG players are so used to these choices already, and many games copy the status effects for convention's sake without understanding what made them work at all in the first place. Like I've played RPGs where you win around 50 gold per battle, one Antidote costs 4 gold, your party can carry 99 stock of them in an unlimited inventory, and the battles are such mindless button mashers that using a turn in battle doesn't really cost anything. In such circumstances, dealing with Poison is about as challenging or interesting as stuffing envelopes.