When you don't use specific knowledge or skills, you may find that, eventually, you just forget said knowledge or skill.

Something embarrassing happened to me the other day. I was playing around with a new server on Digital Ocean and it occurred to me: I had no idea how to manage it. This is slightly awkward because I've been a sysadmin for over 10yrs, the largest part of my professional career.

https://ma.ttias.be/i-forgot-how-to-manage-a-server/