Previously I could browse the store and it would mark those games I own and those I didnt, thus making buying them easier, is this no longer the case?

It still is, but you have to look a little harder.

…Actually, I think they broke something. Give the support cat a nudge.

It's been broken for eons - there are userscripts to plug this issue but GOG really need to bloody fix it.

Yeah, i have the same issue and it happens both at home and at work. The bug seems to be in whatever "search results" functionality the store has (be it actual search, browsing the store or any deal specific page) since all of those have the issue, but the actual game pages do show if i own a game or not when i visit them. This means that when i visit a deal page (or any other games list) i have to open each game page in a new tab if i do not remember which ones i own (...i have close to 550 games here, so i do not remember all of them :-P).

Weirdly enough it doesn't happen for *all* games, very often i'd see a game list with some games i own marked as such but other games that i also own appear as not owned (unless i visit the game's page, where as i wrote above, it shows that i own it).