If Inner Sphere endosteel is obsolete then another technology vulnerable to obsolescence is IS double heat sinks. Unlike single heat sinks, which remain in use for combat vehicles and civilian applications, there's no reason to keep IS DHS around if you can make Clan versions.
I noticed the binary laser was mentioned, but that's described as obsolete at its introduction. It was one those
Jeff Goldblum technologies. Never mind heavy PPCs, which came along far after the binary laser, a normal PPC and AC/10 are pretty competitive with it. You're getting 2 extra points of damage (admittedly reaching guaranteed headcapping levels) for less range and 16 points of heat in eras when single heat sinks were the norm.
Among Clan technologies, I'd say the ATM is pretty obsolete. It's a direct fire weapon, which puts it in a crowded field of excellent direct fire Clan weapons, like the Clan lasers, HAGs, Gauss rifle, and PPC. Most other Clan direct fire weapons aren't vulnerable to AMS like the ATM. Competing Clan missile systems - like the Clan LRM - aren't forced to use 3 different types of ammo to achieve their basic functionality. Clan LRMs also offer indirect fire and a variety of useful alternate ammos, from mines to smoke. Clan SRMs and Streak SRMs are better at crit-finding than ATMs.
Battletech doesn't have any guns that approach the size of an 8 inch cannon,
Side comment: At 200kg per shot, the Long Tom would be around 9-10 inches. At least one Clan UAC/20 was described as 200mm, which means it was only firing a couple of shells per "shot."
