to be fair, Botham bowled pretty well vs Australia in 1985, and threw in there some very useful innings when the series was still up for grabs, but yeah there was a spark missing from his bowling after 1982, bar the odd performance (8 for vs WI being one)
That 8-for would've been great had it not been followed-up by 20-117-0. That sort of thing takes the gloss off something somewhat - same with Hoggard at Christchurch.
1985 was the only time he bowled semi-decently from the Second Test 1981\82 onwards, and even then he was still nothing like as good as he had been before 1981\82.
And with the bat from the second-innings of the Third Test of 1984 to 1989 (I refuse to base anything whatsoever on the Botham of 1991 and 1992, his career should have ended in 1989 IMO) he passed 50 6 times in 44 innings. You could probably go so far as to say he had a really big influence (the type he had for fun in his first 111 innings) on the game once in said period.
I'd say it's stretching credulity to claim Botham was anything other than decidedly poor from mid-1984 onwards. Not quite Khaled Mahmud-esque, but certainly nothing more than a Paul Strang \ Chandika Hathrusinghe type player. Useful of times, but nothing more.
He still had the attitude, of course, and that could go some way to making him look a bit better than he was any more.
IMO there's no better illustration of the two parts of Botham than two incidents in 1981 and 1985. Both triumphant summers, but 1981 saw
the innings at Old Trafford where he slammed Lillee, Whitney and Bright all over the shop for 118 off 102 balls. In 1985 at The Oval he slammed Hughes for 16 in an over... then was caught off McDermott.
Same attitude, same style of play... far, far less effective.