I think the idea is to help 1.23 players update jj2 to the most definitive experience in one go, which is 1.24+. Now for 90% of the use cases that's irrelevant. And my first thought didn't go to multiplayer levels from the era of MCEs that seem incredibly niche. I think only some niche battles and races use that (icboomerang for example comes to mind, but it accounts for both ambient sound IDs I think).

My first thought went to how 1.23 does not have lori, and a lot of entry level tests require you to morph to lori for a level or to morph during the test in a specific order.

The scenario:
Online is empty most of the day. A new player joins the battle server during dead hours and not realise there is peak hours in the weekend or a dedicated discord organizing events.

This person joins the battle server in 1.23. The person is very interested in the game. A veteran joins and wants to bring this person to experience some custom content and wants to bring them to a test server, which needs 1.24+. This means you need to convince this person to install TSF and also plus. The problem is that the new player and the veteran have a huge language barrier.

Why tests specifically? It's a good hook to get people hooked on online play. New players can play it on their own pace and skill level, engage with other people and come back at it at any time regardless of jj2s activity. Out of any gamemodes it has a high probability to get new players to enjoy jj2 and see something different.

If communication is very difficult, any extra step to get things to work might become a difficult task. If asking about Lori, said person might not even know what a Lori is. If asking about version numbers that has low chance of being succesful. If you can just see that person has 1.23- you can focus on giving the other version. Just linking to GOG for 1.24 when possibly the person already has a legal copy of 1.24 installed is immoral too. Perhaps as a veteran test enjoyer you only care about getting more people to play tests, but know that you are more likely to succeed with 1.24 players than 1.23.

Yeah it's an incredibly niche situation but this is the only thing I can think of other than just fear of losing information. To play devil's advocate: why would we not show it, because the likelihood of there being 1.23 players still out there is extremely low, and the people wanting to know is also extremely low. Why would a vague difference like a teal/green coloured icon be harmful for informing the player. Some information is just there for 99% to ignore and 1% to make use of, and that's fine. I think as long as it doesn't have extra icons or takes space, it's unharmful.