The way Tachiyomi worked is that it used specially written extensions to automatically connect to different web sites that provide access to manga and download from them. I mean simply saying it worked kinda like web browser except if you had to download search engines yourself. And in that comparison any legal threats against it looked very dumb. Like someone said "If Warner Bros sued Google Chrome because it gives access to piracy sites". With last update they even completely removed direct access to download these extensions and instead you have to manually provide link to third-party repo with them. I really thought that big change would safe Tachiyomi from any legal trouble for a long time