Its a word play, like incel (being involuntary celibate), but despite the word being a shortening soon enough internet gave birth to femcel. This had the same meaning as incel but for women, or more accurately female. Although the original in theory could be used for anyone it became a more or less 4chan exclusive (as in you are using 4chan -> you are an incel).

That all being context, now a words with a XYZcel structure became used to say marginalized (not necessary, but the person using it believes it true) community with some core identifying mark.

Freedomcel by that logic means a marginalized group of people who like freedom.

Since this is on shitpost, I hope it's not really a thing

Those involuntarily without freedom.