It used to be possible right ? doesnt have to be a strictly youtube focused site either, being able to grab url from other sites are always a plus, only suggest apps that are on fdroid (main repo) .
Also for those who dk and might find useful you can use webtor to get a network stream url from magnet links, tho i wont suggest it as the lag and experience is abysmal and misses important functions like seeding .
its cause tubular uses newpipe extractor for fetching videos and itbwas broken for a few days, got fixed yesterday tho so will be solved when they merge upstream changes, id recommend using newpipe over tubular, its what i use .