The top comments on that thread discuss the importance of owning your email address by way of using a custom domain, so you can move to a new providers and not worry about losing contacts or missing emails. I fully agree with that.

Back in 2014, both Outlook and Gmail had free family plans with custom domain capability, but have since removed those options. Is there a good low-cost option for custom domain email in 2019? For people who use and rely on email a lot, Fastmail is definitely the best option, but it is too expensive for my use case. I would like to set up custom domains for my family (5-10 people in total) and I would like to know what is the cost-effective best practice to do so these days.

One option would be to set up a micro VM at AWS, or Digital Ocean, etc. to host your mail server, with something like Squirrel Mail. Then set up your gmail account to allow forwarding for your other email address ("send mail as" in Settings / Accounts and Import), so that outbound mail from your private server can be forwarded through Google's servers (so you don't get blocked by everyone else).

This way, if Google mail goes away, you can pick up another free email provider to relay mail through in the future (assuming everyone else also allows "send mail as").

Is this how your email workflow is setup?

Yandex email is free, even if you want to bring your own domain. https://connect.yandex.com/pdd/

Zoho has free custom domains. I don't use the web client much, but it's actually pretty good.

Just about all of the big hosting companies will do email hosting. Everywhere from $2/month to $15/monht (including office 365)

you don't even need to buy your domain from them.

This seems to be a good solution. Have you used it yourself? How has your experience been with migadu?

mailbox.org - it allows you to have up to 25 aliases so you can use it with multiple domains

Yandex is free for your own domain. Not sure how many mailboxes it supports.