https://oyster.tfl.gov.uk/
currently leads to
https://oyster.tfl.gov.uk/tfl-oyster/maintenance.shtml

"Sorry - Important site maintenance means Oyster Online is temporarily
unavailable. [...]"

For the past week or so, the Oyster Online login page has displayed
the time scale for this planned maintenance - Friday evening to
sometime on Sunday (I think it was early on Sun morning, but can't
remember) - so it's a bit annoying that this time scale is not shown
on the current splash screen.