Duuuuuude. How far has time passed?!

damn.. what's the TLDR? What is the speculated worst?

TLDR: it was bought, and the new owners inserted telemetry that potentially reported all the extension's user's viewing history back to them.

What the “speculated worst” is from that depends on your paranoia about such information gathering. Most likely the information was to be fed into various marketing databases.

Obviously the same information could be used for even more nefarious purposes, particularly for users in countries with very monitor-y governments and people whose governments otherwise want human-rights abusing controls over them (women in a number of US states for instance – in fact even if their local legislature is not acting on tjhat sort of information, unpleasant groups active within the state might be).