Yes, at the moment, "visa runs" are permitted as long as you don't raise suspicion that you are doing something illegal (working without permit etc.) in China and your intended and actual stay, at each entry, does not exceed 30 days. Hong Kong SAR is considered a separate jurisdiction for immigration purposes. A day trip to Hong Kong is extremely common and normal tourist activity.
Question 10: Are multiple entries allowed?Are there requirements for duration of entry intervals? Are there limits on the number of visa-free entries or total days of stay limit?
Answer: Foreign citizens eligible for visa-free entry to China can enter for multiple times. Currently, there are no restrictions on the number of visa-free entries or total days of stay limit. It is of vital importance to keep in mind not to be engaged in activities inconsistent with your purpose of entry.
http://ch.china-embassy.gov.cn/fra/ls_fw_s_2/sbqz/202403/t20240307_11255418.htm
I took the above from the Chinese Embassy to Switzerland, but you'll find the same on other official sites too. NB: The period of stay has been extended from 15 to 30 days recently (November 2024).