Elton John headlined Sunday, and throughout his set ... there was a lot of chatter online that Britney might, perhaps, come out and perform with him as a special guest. The song everyone thought they'd duet together ... their 2022 collab, 'Hold Me Closer.'

As the show went on, however, it became clear to folks ... BS wasn't going to show up. That became even more evident when Brandon Flowers was trotted out for 'Tiny Dancer.'

The reactions were pretty funny -- people on the Bird app were (jokingly?) up in arms over the fact BF was tapped instead of Britney ... and that all the anticipation amounted to nothing.

It's unclear why they thought she'd be making it to Glastonbury in the first place ... there've been no concrete reports that was going to happen at all.

As for Britney herself ... she just got back from a vacay down in Mexico -- and has been busy posting her usual random content on IG Sunday, providing no real indication she was set to grab the mic or perform in front of thousands of people out of nowhere.

Anyone who's been paying attention knows that's likely not going to go down ... maybe even forever. As we've reported (repeatedly) -- Britney has no real interest in stepping into the spotlight again ... although she is gearing up for a recording comeback.

In other words, all of the fans who are hoping to see her live and in the flesh again are likely going to be let down. Britney doesn't even really venture out into the public anymore.

Another sign that this Britney pipe dream wasn't going to manifest at Glastonbury -- the fact that she's barely audible on 'Hold Me Closer' to begin with ... her vocals are faint and practically imperceptible. Yeah, it was cool to see her hop back into music ... but it was far from the true return to form everyone was expecting. That has yet to be accomplished.