Is MMR Decay a Thing?

My friend group has recently gotten back into league. We've been playing since season 1, but have been inactive for 5 years or so. And solo queue is frankly completely unplayable for some of them. Any higher elo friends find ranked completely unplayable. Even with consistent play and a silver/gold rank, they face off against diamond players. Dozens of games, and its still the case. With that big a gap in play time, their skills have decayed to gold/silver level, and unfortunately their MMR has not matched. Some of them have flat out just quit.

In CSGO you decay significantly one month after inactivity. Rust kicks in significantly faster in CSGO, but at least the game feels playable on return.