I think some threads on mumsnet can certainly provide a climate where transphobic remarks are ripe to be expressed by those who hold them.
Generally however I think the mods do a reasonable job of deleting individual transphobic remarks - but of course there will be a period of time where that content is up, and available to read by anyone.
I sometimes see entire threads pulled where there is robust discussion going on, and I think that is regrettable, but I don't find it too surprising given that they can be peppered with individual transphobic comments and I assume it becomes too hard to intensely scrutinise content the ongoing discussion and they have to make the call.