When did Coinbase begin issuing 1099-K forms to users?

I remember Coinbase sent out 1099-K forms for 2018 sometime in early 2019 to those who qualified (you needed to have more than $20,000 in gross proceeds and over 200 transactions). But what about 2017? A lot of people made gains that year too. When did they actually start sending W-9 or 1099-K forms? I came across a post from 2016 that mentioned it.

The post you quoted for 2018 has a news linked that tells about Form 1099-K being sent to clients for the year 2017 and not meant for taxes on gains in 2018 (news was posted in Feb 2018) and I did took some time to find other sources about this one and Google is only showing me the results of the news link above. If you are worried about missing to file your tax report for 2017 I think you need to check your email because that is where they will send you a copy of the Form 1099-K, if you haven't received any email they have a copy here where you can download it.

I think my question is pretty much moot nowadays that IRS seems to just find out who owns crypto anyway.

Yes, but you have to file it anyways, here is the draft for 2020, https://www.irs.gov/pub/irs-dft/f1040--dft.pdf. Maybe you can take a look at it and prepare for the information that is needed if IRS is going to make this official.