The reason it was canned
It wasn't bc it was sooo bad it couldn't possibly be released like Warners sort of hinted at (seriously? Look at some of the sh*t that gets released!) or tax reasons , or it was too expensive for a stream movie, not big enough for a cinema release BS
It was bc there was a change of DC management which altered the original ending of The Flash which had Keaton as the new Batman of the DCEU visiting Barry outside the courthouse with the new Supergirl (there was also rumours of an ending with Cavill and Gadot but it's not clear if this was instead of or in addition to Keaton/Supergirl), then it was canned for the humorous Clooney shock end (apparently thought up by James Gunn when he took over DC bc he knew he was scrapping the whole DCEU to reboot as the DCU)..
So Batgirl was taking that original Flash ending and having Keaton as Batman in the new altered DCEU (with JK Simmons back as JLs Cmsr Gordon) , a Keaton cameo was also filmed for Aquaman 2 (before being reshot with Affleck when it looked like he was coming back to be Batman in DCEU, presumably AM2 would've been set in the original unaltered DCEU universe), then scrapped entirely (obviously bc now Clooney was effectively the new Batman of the altered DCEU and they weren't going to do a AM2 cameo or reshoot Keatons Batgirl scenes with him as his appearance was just a one and done fun scene for The Flash when Gunn took over DC)
So Batgirl just didn't fit anymore (was supposedly to come out after Flash, before AM2) it was a victim of shifting multiverses!