Lawyers can't stop you from creating your own spin off. They can only prevent you from publishing & selling it.

I don't expect anyone here would be trying to start up their own physical print book sales or physical hobby model sales, though. The threat of legal issue is pretty much just a cope to avoid doing the work.

I'm not a BattleTech fan so I don't have a handle on the nuances of the setting, but it seems fairly straightforward to clone since the setting seems to only have one major conceit (big manned robots).

Creating a setting with some names-filed-off factions with new characters would be easy. For copyright-safety you can't use literally-the-same mech designs & names but there's enough mech designs and material out there from various properties that it would be very difficult to claim something is a infringement rather than a derivative work.