I think major issues are fairly common, about once or twice a year if you look across all banks. The biggest ones I can remember are TSB and, about six years ago, there was also a software update at RBS that broke their banking platform and had to be rolled back due to a mistake in the code.
Because it had been pushed to production, there was then a huge backlog of transactions which took them days to clear before all customers had a fully updated account.
More regular downtimes are either planned maintenance (so some warning is usually given as Barclays do with the clock change) or issues with capacity where only, say, 50% of infrastructure goes offline rather than a full blackout (so it still works but is buggy or requires a few tries to do things).
Of course, there was also the famous Visa outage.
Finally worth a mention is that scheduled downtime is often needed to launch new features at traditional banks due to the way their systems work. This meant that many had downtime to prepare for open banking, or make changes for ringfencing, etc