Anyone willing to shed some light?
Opinion Required : OTB Rule Mode for illegal moves in Online Chess

A player who relies solely on the website to tell which moves are legal or not, must be extremely low rated. Someone who tries to calculate even 1 move ahead must know which moves are possible and which aren't.

I disagree, even at gm level and under pressure, players make illegal moves. And at beginner level, players have this tendency to overlook pins, or may move their kings in check, may perform castling when the squares were king is to move are under check. My point is that why can't we align the repercussion for similar mistakes committed in OTB and online chess.

At the GM level, illegal moves are one in a million. Even at the 1500-1800 level they are extremely rare.

So you're saying that the allowing of illegal moves teaches a player more than an computer alert? I don't think they're all that different.

Also, beginners are making illegal moves because they are inexperienced (and occasionally are unsure how the pieces move at all). Showing them which move is possible and which isn't is a good thing, not a bad thing. Suddenly losing game after game just because you played an illegal move would be extremely frustrating and would serve no purpose.

I agree, the feature serves a purpose. But the point I am presenting is about having an addition option, which a player can choose at the beginning of the game, if they want to align the online experience more closely to the OTB experience. It is not forced upon, nor default, but chosen by someone who wants to choose it.

Moreover, would the opponent require a report or spot button to report like otb as most otb games the arbiter might not intervene if both sides don''t spot it

Additional information: FIDE regulations on online chess (for fide rated tournaments i think) require disallowing illegal moves

Well, I think I agree with most of the point raised, which outweighs the benefit of implementing any such option. This raises another question, how different the game becomes if we have to keep our eye extra open for ilegal moves vs when we know illegal moves are impossible. The difference won't matter much for a high rated player, but definitely for a lower rated player.
The cushion against the repercussions of playing illegal moves in online chess makes learners weak at noticing illegalities during calculation, as they simply rely on the game to alert them. This habit builds sloppiness, especially when transferred to over-the-board (OTB) chess, where no such automatic safeguard exists. To encourage better learning, there should be a game mode where players can opt for stricter OTB-style rules: the first illegal move results in a warning, and the second illegal move results in disqualification.