Two of the other replies are wrong. This isn't actually about the new 18+ age verification stuff that countries seem to be ramming through right now - as far as I know, Discord uses third parties for that service. The link from Discord's statement in the article mentions that this is about appealing account bans of users who were suspected to be under the legal age to use Discord at all (<13 in most places). This is an older thing, which also explains the amount of data that was leaked.

Joining "NSFW channels", which usually means porn. But some normal channel are also tagged NSFW to opt out of Discord's forced content filter on public servers, which has occasional baffling false positives.

Online Safety Act for the UK. You will be safe.

As the article says it’s used for age verification