The sad truth is, nobody bothers to add support for RSS if they write a new custom blog engine.
I've started to keep a list of all the websites that I've tried to subscribe but didn't support RSS:
https://angel.co/blog
https://blog.close.com
https://blog.messagebird.com
https://blog.twitter.com/developer/en_us.html
https://blog.twitter.com/engineering/en_us.html
https://bloggingfordevs.com/blog
https://business.instagram.com/blog
https://developer.spotify.com/community/news/announcements
https://discord.com/blog
https://frontapp.com/blog
https://indiehackers.com
https://joelcalifa.com/blog
https://linktr.ee/blog
https://mixpanel.com/blog
https://openai.com/blog/openai-api
https://paystack.com/blog
https://segment.com/blog
https://telegram.org/blog
https://transferwise.com/gb/blog
https://web.dev/blog
https://www.anyscale.com/blog
https://www.binance.com/en/blog
https://www.bugsnag.com/blog
https://www.changeinvest.com/blog
https://www.hackthebox.com/blog
https://www.integromat.com/en/blog
https://www.linode.com/blog
https://www.loom.com/blog
https://www.makerpad.co/blog
https://www.tradingview.com/blog/en
https://zapier.com/engineering