Can foreigners work from home in Thailand under a Tourist Visa?

Working without a valid work permit, even for one day, is a criminal offense. The penalty for working without a work permit is a fine of THB 5,000 to THB 50,000. This is to be followed promptly by deportation. They may be blacklisted and not allowed to reenter Thailand. A company or employer that hires a foreigner without a valid work permit is subject to a fine of THB 10,000 to THB 100,000 for each foreign employee. An employer committing a repeat offense is liable to imprisonment for a term of up to one year, a fine of THB 50,000 to THB 200,000 per foreigner employed, or both. The person will also be prohibited from employing a foreigner for three years from the date of final court judgment.

If requested, a work permit must be presented to the officer within a reasonable time frame.

/2019_Thailand_Legal_Basics.pdf

I think any work from home situation you are talking about will not be a concern to immigration. If you are working on the internet or some remote office situation. If the fruits of your labor are realized outside of Thailand, then immigration will not likely care.

If work from home, like open a restaurant and sell cheese sandwiches to people here in Thailand, probably Immigration won't care, but it is easier.

If work from home, like have an employer here in Thailand, get a paycheck here and Thailand and teaching kids in Thailand, then immigration probably wont care, but some government office, some day, may want headlines so they'll do some stuff but probably nothing to you.

If work from home you been, go to the office in Thailand, for a Thai employer, the fruits of your labor are realized in Thailand. Still immigration probably wont care so much, but the police will want headlines and they will come arrest you. Then immigration will boot you.

They've been converting existing visas and extensions to non-B/extensions of stay for employment in country, including for those you previously went overseas for.

This is pretty much in the hands of your employer as they'll need to provide more information than if you applied overseas but it definitely is possible.

No. You are not allowed to work unless you have a work permit issued. That may be from a non B visa or a non O visa based on marriage to a Thai citizen. Covid hasn’t changed that.