You're right, these things are bound to encounter some older-style light guns, were it only in the game, that is what I pointed out saying that simulating slat will have us screwed up both ways. Consider just that there is one small (small catergory of weapons behaving not-quite-accurately regarding reality) and one big (not taking SLAT armour into account at all) drawback.
Since no one of us speaking here designs the game himself, we plain players have to make compromises...
What I meant is that even though old 90mm guns have nothing better than HEAT, some actually do have. And even old gun can be fitted withmodern ammo, which is way cheaper than building new tanks. Look at the new sabot rounds for the T-55 guns, for one thing.
Anyway, those old rust heaps will be stuck against modern far-reaching forces, so what? Isn't that the point? As you point out, at modern combat ranges, they will have barely a chance of scoring a hit, let alone a kill.
Look at it this way: you have some Red forces with old M-47 or M-24 or anything facing some Stryker-slats integrated to a modern task force. At long range they will blaze away at the Strykers without doing any harm and get busted in one turn. Anyway at 'Javelin range' their HEAT round will have lost so much KE that they will fal off and should et stuck in the slat anyway.
At short range, they will have more use of their AP rounds, crappy as they might be, and could be able to bore through all slat, composite, ceramic, stell, spall liner or whatever.
So you have both sides at top realism!
And you can also consider the cae of top huts, against which the slat is no protction at all, and which happen more frequently than one may think.