https://www.bbc.co.uk/news/articles/cw4dd9lzx02o
There was a policy in place at this year's Eurovision to only permit people to bring in the flags of the 37 competing countries and the rainbow flag. This meant that it was not permitted to bring in the EU flag.
This policy also meant that the eventual winner Nemo was not allowed to bring in the non-binary flag, and had to sneak it in. Ireland's Bambie Thug wore a sort of trans flag bikini(? I don't know clothes) for their performance.
A spokesperson for the EBU said that this policy was the same as in 2023, but from what I've heard it wasn't as rigorously enforced in Liverpool. Every year there is a flag policy for the venue. Some years it's just specific flags being banned, e.g. that of ISIS.