I wanted to response to the issue, as it looks a rather important response:
I've possible found the place where the data was stolen. (I can't say for 100% sure.)

In 2021 their was an information leak on twitter and the data of 200 million accounts was stolen. This information was leaked earlier this month (in the last week or so, I think) Most people will still remember the panic from back then.

Another suspicious candidate is also from 2021:
In March 2021 their was a data breach on Mangadex and data from around 3 million account where stolen.

Here is a list off recent leaks and hacked (I've copy paste this from a search engine specific for such problem, given by my newspaper in the past.)

Recently added breaches
DoorDash: 367,476DoorDash accounts
SlideTeam: 1,464,271 SlideTeam accounts
Twitter (200M): 211,524,284Twitter (200M) accounts
Deezer: 229,037,936Deezer accounts
Benchmark:93,343Benchmark accounts
Gemini:5,274,214Gemini accounts
CoinTracker:1,557,153CoinTracker accounts
Abandonia (2022):919,790Abandonia (2022) accounts
Not Acxiom: 51,730,831Not Acxiom accounts
Get Revenge On Your Ex:79,195Get Revenge On Your Ex accounts

I've never heard about most of them, certainly the last one is weird.
Am I allowed to post this site over here, that gives this information?
The good news: Mangaupdates isn't on any of those lists.

Update:
If the accounts where older than 2016, then it's possible from Anime-planet from the breach in 2016

Other manga related breaches: Mangatraders (2014), Mangafox (2016), Mangatoon (2022) and
Nihonomaru (2016)

Even after dead, we raise from the ashes for the sake of manga.