The design drives a player to use their abilities or lose them.

In game if a player is not using an ability regularly, they forget the ability completely and it is no longer accessible to them. Maybe they have to return to where they learned it, or a skill tree degrades without use.

Specific bonuses to combat skill degrade without being in combat for extended periods, or a combat mechanic or spell becomes forgotten.

There could be a training center to relearn what was lost. Or they just never get it again and really drive the need to utilize their skills, integrating all their abilities into their use rotation.

Is it annoying? Is it realistic? I love the idea for some reason, as a lot of games I just sit on a mountain of abilities though I use a specific set that just matches how I want to play.

Perhaps this drives the opportunity to learn different skills and the player ends up deliberately avoiding use of certain abilities.

A timer could kick off when a new ability is learned and if the ability is not used a certain amount of times within that timer, the ability is lost. The time would repeatedly reset, and perhaps get longer with regular use of the ability, with a well known ability being the hardest to forget.

Maybe there is a threshold of ability use where the ability becomes inherent and is now permanent. Encouraging regular use to truly master it and making that accumulated pile of skills more valuable and better known in application.