CSRS ended in 1983 or 1985 I forger which.
Very few CSRS’ers left.
You are sort of correct, CSRS eligibility ended in 1986, and there was a half-hearted push to get civil servants onto FERS, but as of 2018 (the best figures I can still find), they were still 6% of the workforce.
5 or 6 percent seems like a small amount, but that small number disproportionally represents senior decisionmaking roles. That's not necessarily bad on its own, but it does make dislodging bad bureaucracy and bad bureaucratic decisions much tougher. If you want to drain the swamp, one way is to disincentiveize staying in it. Also, the ones who rose to a certain level and then just stuck it out, they are holding younger folk back.In agencies and departments legislatively bound to have no more than 3% of their workforce above a certain pay grade, a GS13 who has been spending every morning doing a crossword puzzle and half the afternoon napping represents one the spot a much more competant GS 12 can't have.
I know there are a few folks on this forum who would be fine with every federal employee being miserable and getting nothing done ever, but you don't get good government by having a miserable and lazy workforce, and you don't get good senior level workers by having your best junior workers quit federal service in disgust because a guy who can't open a PDF file on his own is managing an IT department and mocking his millennial underlings for not knowing how to sew on a button.