In game, I have the Season 8 Power Bonus Triumph stuck in my "Featured Triumphs" list even though it is no longer earn-able/complete-able:

via Reddit, found an in-game/app image ^ of the situation, in case some aren't familiar

Like that tiny little grain of sand in your shoe that you can't seem to shake out no matter how you try, Season 8 Power Bonus has been there, featured, all through Season 9 and will continue to be there beyond it, I'd imagine?
I don't take issue with the fact that this triumph has not been hidden from my view. (I think that is the wrong solution) I actually prefer seeing ALL Triumphs/items I'm missing, rather than having them just disappear from Collections and removed from the already inexact equation.

Along with all the other changes of a Shadowkeep sized expansion, as well as new mechanics, it made getting up to speed in Season 8 take an extended time. I know I'm not the brightest bulb on the tree and it takes me extra time to acclimate to a new play-flow. Compound that with losing 14 days right out of the gate and Season 8 comes up feeling short.

By the end of Season 8 I was able to reach Season Rank 177
Whereas in Season 9, I hit Season Rank 177 with 28 days remaining to play

Season 9 has moved along quickly and maybe progress has been artificially bolstered by The Dawning bounties, Crimson Days, and Empyrean Foundation donating/bounties? (though I have not utilized the latter)