UPDATE: As of right now the 14 quarantine has been canceled (never actually started).
foreigners and Thais returning from Japan, South Korea, Germany, China, Taiwan, France, Singapore, Italy and Iran, Macau and Hong Kong are subject to a 14 day home quarantine REGARDLESS of symptoms, effective immediately.
Foreigners without a permanent residence (tourists) MUST show immigration PROOF of where they will be self quarantined at for 14 days or they will be refused entry.
TLDR : Answer Summary
As of now, the 14-day quarantine requirement has been canceled but, effective immediately, foreigners and Thais returning from specific countries (Japan, South Korea, Germany, China, Taiwan, France, Singapore, Italy, Iran, Macau, and Hong Kong) are subject to a mandatory 14-day home quarantine, regardless of symptoms. Foreign tourists must provide immigration proof of their quarantine location or face entry refusal.