Google’s Gary Illyes said on Twitter that Google will eventually remove pages from the index if they show HTTP status error codes such as 429 or 503 for a longer period of time.
Fun fact: serving 429/503 http status codes will slow down crawling, yes, but if you serve them for a longer period of time, they will remove the pages served with said codes from the index. pic.twitter.com/p4Wf6YD6SX— Gary "鯨理／경리" Illyes (@methode) February 20, 2020
You can use the website audit tool in SEOprofiler to check the HTTP status codes that the web pages on your website send.
Check the HTTP status codes of your web pages
Among many other things, the website audit tool in SEOprofiler checks the HTTP status codes of your web pages: