Weren't your customers annoyed?
I was also sixteen when I did that, so I mean, of course I wasn't going to do anything serious with it.
Google and others will happily block your mail or send it to spam folder even if you never sent one SPAM email ever, and have all those technologies you mentioned set up.
Gmail et al have been spam filtering messages from correctly configured mail servers for a decade+ now. All the dkim, dmarc, and spf in the world won't help you if you aren't known to them.
Remember, even if you have everything 100% nailed down with SPF, DKIM, etc. you can still end up on a random blacklist, some of which are basically extortion shakedowns. Now, you can say, "well ignore those losers, who cares?" but sometimes you have a customer who directly or indirectly relies on those blacklists. I certainly do, that's how I found out!