I disagree, it would add a lot of flavour to medium tank engagements as well. In the case of mediums like P4 Ausf.J or Sherman EZ8s now winning against cheaper tanks purely because of the armor advantage, with side armor they would actually have to be microed well or they could easily go down to these cheaper tanks. Even for light tanks it would make engagements more dynamic, as for example 2x 222s would stand a much better chance vs a Stuart, and AT Rifles wouldn't be as leave in cover and forget.
ATGs could probably get a lot less penetration so they'd have a good chance to bounce off even most medium tanks' front armor, only working well when positioned on the flanks. ATGs in CoH2 are arguably a bit too powerful (making stalling strats or comebacks too viable) as almost all of them are guaranteed to penetrate mediums frontally.
Micro would become absolutely vital, and it will allow cheap tanks to become very cost effective when used well. It could also help balance tank destroyers better against mediums, as for example a vehicle like the Jackson could get lower penetration so it'd have to use its superior mobility to get side shots, rather than easily shutting down both medium and heavy tanks frontally from 60 range because of high penetration. Heavy tanks wouldn't have to be gated behind artificial delays (such as high CPs) as they would be a lot more vulnerable to an overwhelming number of flanking mediums taking side shots. All this would make medium tank play a lot more viable.
I've played a lot of Wargame (and some Steel Division) and having side armor on all vehicles really helped making tank combat very diverse, dynamic and exciting. It made all strategies (going for expensive main battle tanks, or going for a lot of medium main battle tanks, or going for a healthy mix, or even going for a swarm of cheap outdated tanks) viable as long as they were microed well.
I’m a huge fan of side armor and armor angle becoming a feature of CoH3.
It poses a huge opportunity for added realism in a way that is still easily understandable to they player at a glance. It would also make for interesting historical tactics more viable and give distinct advantages to some units.
For example, Panthers have very strong frontal armor, but weak sides and rear. Tigers have strong frontal armor and good side armor, so positioning one at a 45 degree angle to incoming fire gives it an angled advantage that only works with tanks that have good side armor (real historical tactic taught to Tiger crews while Panther crews were told to never do this due to having weak side armor.)
Sherman tanks also had surprisingly good frontal armor even compared to a Tiger (3.6 vs 4.1 effective frontal armor value when you factor in slope), but they had flat sides that were quite thin by comparison.
Panzer IVs has decent front armor but weak sides and rear and, to compensate against AT rifles used by the Soviets, they added side skirts. This could translate to Panzer IVs having bonus side armor only when equipped with side skirts, because it’s frankly silly that side skirts add frontal armor, as it does in CoH2, when it is clearly only on the side of the tank. (An example of how current visual indicators in the game don’t match with bonuses and game mechanics).
Side armor also gives players a tactical choice when engaging, examples are Panther vs Sherman and Sherman vs Panzer IV.
The Sherman wants to flank the Panther to get at his side armor but to do so would expose his own side armor and so does the Panther. Meanwhile the Panzer IV with his skirts wants to flank the Sherman to get at his side armor and has stronger side armor himself so it’s a less risky maneuver for him. In this matchup the Sherman wants to keep it as a frontal engagement where his superior front armor outmatches the Panzer IV, but not the Panther.
Really it just adds a whole new level of tactical complexity that is easy to understand, but hard to master.