Several big things about this are extremely annoying to me.

1. I use Gmailify at the moment in order to sync two different Gmail accounts so one is a complete backup. I prefer having an online backup so it would be easily accessible no matter where I am located.
2. I use POP with Gomailify (a different service than Gmailify) for some domain email, which I believe is currently required by Gomailify. Currently, Gomailify won't start working until they detect the first POP attempt from Gmail. Gomailify also forwards my domain email, so hopefully they will work out a new system so POP is not required.
3. I also use POP to bring other domain emails into the web version of Gmail as that is currently the only way to "check mail from other accounts." Apparently, they are suggesting we will be able to add IMAP accounts to the Gmail app, but will that email then show in webmail Gmail? I imagine so. I will have to test that out.
4. I suppose we could just forward our emails from other accounts to Gmail, but there are several problems with forwarding. It probably would not count as "activity" on the other accounts to make sure you don't lose access due to inactivity. Also, there are problems from time to time with Gmail deciding an account is a spammer and blocking it if too much spam is being forwarded.

It strikes me as very odd that Gmail no longer wants people to use Gmail as their central email interface where email from other accounts is read and acted on.

This is true. Forwarding is not a solution in many cases.
- It does not qualify as an "activity" for extending "inactivity period"
- If forwarding failed for any reason, you do not know this

Regarding Gmailify: it has limited capabilities for filtering. Even these capabilities will be removed according to Google announcement.

Its interesting Google just blocked thier search engine from older browsers a few days ago......

Im really surprised they allowed older browsers as long as they did... They seem to be quite determined to spy on people as much as they can!!

Google seems to be embracing and approving of forwarding now based on their help docs. Maybe now that SPF and DKIM are being enforced they find forwarding is not a problem. When I forward to Gmail from other accounts I keep the originals in the Inbox and periodically I skim them to make sure I haven't missed anything important. Frankly, I have found forwarding very reliable and much faster than either POP or IMAP.

Any alternative solution/provider?

FYI as of this morning my Gmail account on the web (not the phone app) still shows the option to check mail from other accounts using POP3. I didn't go through the whole setup process, but I started it for an email address I have outside of Gmail and it looks like it would work.

Replacement to Gmail POP3 Fetcher: https://postdirect.net/mailbox

$15/year

It fetches emails via POP3 and forwards them to Gmail or any other email address.

Still better than alternatives.

Another option is Fastmail that can be used as a POP3 fetcher and then to forward to another email address.

Another option: https://magicforward.email/

Gomailify has rolled out their new system using a Google API to link to your Gmail account, meaning even better performance and no use of POP3 fetching or forwarding. You need to sign in using OAUTH. You still get SMTP for sending, a new dashboard, unlimited aliases and domains, and fast customer service for $12 per year. They grandfathered in existing customers and are adding a free month to existing subscriptions. Very slick. https://www.gomailify.com/

Thanks for this info, I was trying to figure out what to do about this situation. The company that hosts my domain can do forwarding to gmail, but I didn't know if doing so byspassed their spam filtering (like for example, does it forward the email FIRST before running it though filters etc).

But the other issue I was worried about was when composing an email and wanting to send from the domain email address instead of @gmail.com

When How another GMail address on same Google account?
 

I thought I saw some online articles on this, and thought too a mention occurred on these forums, but I still am finding nothing current. Will or DOES Google allow account holder to create an additional GMail address or two on the same Google account?

Understand, I do not mean "Plus-addressing"; and I do not mean "@googlemail.com" instead of "@gmail.com". I mean, new address or alias for real.

You can now change your Gmail address to a new one and you keep the old one as an alias. https://support.google.com/accounts/...l_keyword&rd=1

Thanks, so according to the message there, it is not yet available to everybody, but is being made available for some people.