"...A small change in regards to participation has also been introduced: From now on you have to do damage in a bossfight in order to receive participation, so e.g. healing your teammates is no longer enough. Please note that you have to actually do damage to the boss, with one point being already enough, however. If the boss's resistances fully negate the damage, your attack will not count towards participation."
Source: (News) Dec 12 2019 - Carpets and Participation
Q: Is healing your teammate not a factor in the participation system anymore? Or does it work like before, but you need to deal damage to the boss for the participation to take place?
A: The latter, healing contributes towards participation, but to receive participation in the first place, you have to do damage now at one time in the battle. You don't have to do damage first in order for healing to count towards participation. As long as you have damaged the boss at one point before it dies, all your healing will be taken into account and count towards participation.
Source: (CM response) Ticker Messages December 2019