I'm having difficulty (most of the threads date back to many years) confirming if there is any validity to the "180 days/year rule," for those using tourist visas and visa-exemption stamps (in 2019/soon to be 2020). I know it's not an official rule, but is there any validity at all to it?

No

As you said yourself, it is not an official rule. Still IO's will use it, unofficially.

Not an official rule but if you live more than 180 in a country you become a tax resident.

The 180 rule was removed from the law a good few years ago.   2008 ????? Unsure of exact year.

No more validity than that immigration officers use the rule when they please. Your choice is to talk back to them of accept your faith.

1 hour ago, overherebc said:

The 180 rule was removed from the law a good few years ago.   2008 ????? Unsure of exact year.

That was for visa exempt entries. It was the 90 days in 6 months rule for them and it also started the 15 days at land border crossings. The 15 days rule was rescinded when the 2 per calendar year limit at land border crossing order went into effect on December 31st 2016.

As far as I know there has never been a 180 day rule for tourist visa entries.

That unofficial 180 days rule, is it per calendar year or the last 12 months?

 

(In first case, we would hear stories of people getting questioned for long time stay on TV or VE only after June each year; I did not follow Thaivisa closely enough to judge for myself if that is the case.)

16 hours ago, ChristianPFC said:

That unofficial 180 days rule, is it per calendar year or the last 12 months?

 

(In first case, we would hear stories of people getting questioned for long time stay on TV or VE only after June each year; I did not follow Thaivisa closely enough to judge for myself if that is the case.)

If the IO does not like the look of you or having a bad day they can make up any rule to deny you. There are reports of people being denied after only being here 90 days in a year not even close to the 180, if you are worried about being denied then fly into chiang mai or enter by one of the friendly land borders instead!