I noticed that all the character boost tokens I previously had on my account from past expansions are now gone.
I did not play Shadowlands, but at the end of BfA I had several on the character screen (at least 4). Now I seem to have a single token for a level 50.
Does anyone seem to know what has happened to them?
I still show every boost that my account has received as none have ever been used. The previous ones (all now level 50) are all stacked together and show 4 as their count. The newest one (level 60) is a standalone token.
If you purchased WoD, Legion, and BFA then your Token count should be 3 for the level 50 ones. If you did not purchase Shadowlands, then you would not have a Token for it.
They might have been updated to the newer versions.
I thought I might be wrong, but unfortunately this is not the case. I have one token for a “Shadowlands Boost” and nothing else.
I can’t upload a screenshot or I would.
If you lack the ability to directly post links (requires Trust Level 3), then highlight your URL and click the </> icon above where you are typing.
That will allow the whole URL to be posted (albeit without the ability to click it directly). It will look similar to this:
www.worldofwarcraft.com
Then anyone wanting to visit the link can merely copy/paste your link.
(https://tinypic.host/i/token.K88wq)
1 Shadowlands boost, though I’ve never purchased it, and the previous expansions missing. 
When Shadowlands was rolled in to the base game, all prior expansion boosts were updated to 50 so that you could get started immediately to get to 60.
Your boost was probably upgraded. If there was more than one, then I’d probably suggest a ticket to see what happened, because of this comment in the support article:
World of Warcraft: Shadowlands Heroic and Epic Editions include a Level 50 Character Boost. If you have any unused boost from a previous expansion, it was converted to a level 50 boost.
Perhaps it’s a bug where the conversion only gave one instead of multiple.
Here’s the support article: