Sevi 7
Semi-retired
If you want to know how viable something is in the meta then you should use the viability rankings (aka VRs). Even for old gens, every usage-based tier has one and are updated (usually) annually, if not more.So since no harm is done when Pokemon move between UUBL and OU, is it fair that some Pokemon move between them to more accurately represent the metagame today, this helps newcomers understand what threats are important to a metagame, besides if OU stands for Overused, and some of the Pokemon in that tier are not Overused, then should they be allowed in that respective tier placement?
You can't use tier placement as a means of viability, because one's tier placement and where they are in meta are not well correlated at all. Some BL mons are absolutely useless in in the higher tier, but are there because they're too good for the under tier. In some cases, like with ADV, mons are technically viable in the tier but are not relevant to the meta at all and are ceremonially placed into BL. However, this is not how it works a majority of the time. Furthermore, some mons are viable in upper tiers, but aren't part of the meta enough to make it into the higher tier, even if the tiers aren't completely set in stone. You can see this with RBY Aritcuno, who is a viable mon in OU, but is not meta relevant enough to get out of UU.
Even in Gen 8 tier placement isn't always a good indicator of viability. Ditto and Gastrodon are solid enough mons in OU, but are placed in RU due to usage, and Gengar and Aegislash are in UUBL, but are good in OU.