To be fair, if we're pulling up ancient history here, sending mail wasn't really a problem in the early days. I'd wager that we started throwing more time and resources at email handling beginning about 10-12 years ago. Prior to that, you could just slap together a mail server and off you went. I don't even think we started adding SPF records in DNS until about 2009'ish?
I've been running forums since the vBulletin 2.0 days, starting in 2002. You are completely correct in that it was dead easy to host sites and never having to worry about email back then.
In the last 10 years I've had to spend increasing amounts of time and money ensuring my mail deliverability is as high as possible.
I make my living from running my forums - it is my sole source of income and I also have employees I need to pay, so making sure things work is critical and having emails not get delivered is something I work really hard to avoid.
I use SparkPost for this reason - it took a lot of effort to make sure my user list was cleaned when first starting, and I even wrote my own addon to allow me to differentiate between transactional and non-transactional emails (which does matter!) and to automatically handle bounced emails.
I also pay for a dedicated IP address, which I spent a lot of time carefully "warming up" so its reputation was kept really clean - that process took months to complete. The downside to dedicated IP addresses is that you do need a fairly high sending volume for them to work well - around 100K at a minimum.
I regularly monitor my sending reputation and keep a close eye on factors which may be affecting my deliverability.
It's not enough to have SPF / DKIM / etc all set up - you also need to make sure you clean your member lists and actively disable user accounts that bounce emails. Ensuring that none of your sites are behaving in a "spammy" manner is also critical.
Being on shared mail hosting is about the worst thing you can do - unless the provider is extremely strict with keeping things clean across all users. Still, you have no control over the behaviour of the other senders and their actions can negatively affect you as well.
One thing to note - as much as Microsoft is one of the most difficult providers to work with because they have really strict deliverability requirements which senders often fall foul of ... there is one provider who is actually much worse IMO - Google Gmail!
It's not widely understood that there is a huge difference between how Microsoft and Gmail handles poor quality senders.
Microsoft will bounce emails or delay them when reputation is poor - so it makes it look like they are really difficult to work with (which they are).
But Google will simply discard them silently - not bothering to send back bounces when deliverability becomes too problematic. This becomes a huge issue because you won't be aware that they aren't delivering emails - unless your users explicitly tell you they are having difficulty.
I'm dealing with this issue at the moment with a site I manage for my son's Athletics club (unrelated to my own sites) - I had set up mail forwarding using a hosted forwarding service so that emails to their domain get forwarded to corresponding gmail addresses the club had set up - to avoid having to pay for Google Workspace or similar. We just found out this week that emails haven't been getting through for the past couple of months - but there's no record of failures, they are simply discarded silently by Google and we only found out because people were contacting us via social media and other means, asking why we weren't responding to them.
This is not the first time I've experienced this silent discarding of emails by Google either - so it's not a recent thing. It doesn't seem to happen frequently - only when sending reputation is particularly compromised.
Anyway - the whole point of this is that it is a lot of work to maintain good sending reputation, just setting up SPF & DKIM is not enough.
IMO - the biggest mistake you can make is to not block emails from sending to accounts who bounce emails or unsubscribe or make spam complaints. Continuing to send to these email addresses is going to destroy your sending reputation very quickly.
Your mail content also matters - mail providers actively scan your emails for spam-like keywords or links. Never ever EVER include affiliate links in your emails - this will get flagged as spam very very quickly. Avoid URL shorteners too - especially Bitly, this is a negative flag used by Google especially.
Here is a list of rules suggested by SparkPost to maximise Hotmail/Outlook deliverability:
https://www.sparkpost.com/blog/6-rules-maximum-outlook-deliverability/
