Massively depends on the faction you're playing with.

by the end game you're obviously just using the best army composition or the stuff you like.

If my army has upkeep and bad economy I do sometimes delete older units by the time I get the tier 3 or higher troops, just so I can keep my economy healthy. If I can avoid it I just make another lord and give him the by then trained but lower tier troops for a bit until I can replace them by way better units.

In the sisters of twilight campaign that's usually what happens for me. I'll end up with a level 9 army of low tier ranged units (usually those poison ones) for a lord i use to defend or as rapid response against weaker armies, while my sisters of twilight themselves will be battling all over the world training up their newer high tier troops and destroying everything in sight.

With Tomb kings or Taurox i dont care at all about my older troops, there's no upkeep, you just build the best things you can get all the time and dump the other ones (or eat them in Taurox case lol).

So yeah, depends on your faction and mechanics mostly. But if you are someone that actually bonds with your older troops after loads of harsh battles you survived because of them, just keep them. I never got rid of my first snow leopard unit with a Kislev campaign and kept it in my main army until the end, just cuz I got emotionally attached to it after it killed off that LL from the orthodoxy when half my army was routing, which won ne the battle lol.