Hi Joe, you may have forgotten how to play the tune but I’ll bet you haven’t forgotten everything that was taught in those lessons. There is always a lick or a phrase or roll pattern that you’ll retain.
Some of the tunes I learned when I started out 12 years ago and haven’t played in 8 or 10 years I can play note for note, Where as tunes I learned 2 months ago I struggle to recall. It’s all still in there in my long term memory I just struggle to recall it in the moment. I have heard @BanjoBen say the same thing, things he taught years ago he would struggle to play now.
The way to retain it is to keep practicing it this can be hard as you learn more and more. At this time I have learned about 80% of the lessons Ben has taught on this site plus a ton of other lessons from other resources, I can’t instantly recall most of what I have learned but I can recall some of what I’ve learned.
Some tunes are easier to recall than others. I try to record most of the tunes I have learned and post them on my YouTube Channel as a reminder of what I have achieved on my journey.