Just to chip in which how I experience and interpret this issue, why its a problem for me and the listed solutions don't really work.
I work with many clients, often with many people from an organisation. Often I can't exactly remember the domain the sender sent a message from, or who sent it.
for example, I work with the company, lets call them Pear. I deal with maybe 5 people from that company. Their domain is something odd like, pear-comms.com - something it's tricky to remember when you also deal with 100 other companies. I also can't remember exactly who sent the message - all I know is that, sometime in the past, I got a request from their organisation.
I used to be able to type "pear" in the search bar and it said sender-contains:pear and I could see all the emails that came from that company. Now I need to find out what the sending domain is from that company is first, it's a pain.
For example, what prompted this search was searching for an email from the local university in my town, named for the town we are in. If I search for the name of the town, I get hundreds of results. The autocomplete contains a lot of TLDs, none of which are the University. I need to go and find out what the actual TLD for them is, which either means searching online or scrolling through my emails, of which there are many.
I don't understand why a feature like this would be removed. it's very odd.