It mostly just has to do with how much bigger 1.12.2 is and the forge coding isn't perfect, so yes adding mods really causes the memory leaks. For 1.7.10, I knew you didn't support it which is why I don't want to ask you to fix it, but it's my real only option to use it in 1.7.10, so I was wondering if I could fix it just for my own personal use.
If there are memory leaks to have to be connected with a certain mod. It's not a general problem of 1.12.2. There are many mods for 1.7.10 which cause them as well (one of mine did so too). It's of course your choice to use 1.7.10, but then you have to accept that it's not supported by modders anymore.