If everyone's responsible, no one's responsible. It sounds harsh, but it's true.
You need someone to actually own accessibility. Not in a "point the finger when something goes wrong" way. I don't think finger pointing works well. I'm talking more in a "this person makes sure it happens" way.
That may very well be the developer implementing the feature. Maybe it's the designer who gets it. It can be a subject matter expert (SME).
Hey, maybe it's you!
Whoever that is in your team. They're usually the ones who ask the awkward questions in sprint planning: "what about accessibility?" They check if that new feature works with a screen reader. They spot when accessibility's about to fall off the priority list and pull it back on.
Accessibility can't just live in one person's job description. Your product owner makes sure everyone knows their part. Designers consider contrast and focus states. Developers write semantic HTML. Testers test with assistive tech. Content writers think about plain language.
Add it to your definition of done. Make it part of design reviews. Bake it into acceptance criteria.
What's important though is to build it into your ways of working. It rarely works if you have to change your ways of working overnight. It's always easier to fit it in.
But without an owner, without someone to make sure it does fit in, accessibility becomes the thing everyone assumes someone else is handling.
And that's when shit slips through.
Ownership is the second thing on my list of 10 things you should focus on.