I believe Troon’s link disproves that claim; it’s clearly documented.
BTW, here’s another way to get the value. It uses the space character in the data to serve as a means of splitting it into items of a list. The desired item is the second one in the list (99) which is identified by an index value of 1 because lists are zero-based.
value_template: "{{ value.split()[1] | int }}"
NOTE
The assumption it makes is that the received format is consistent and the desired value is always the second item in the list. However, if the received data’s format is inconsistent then the technique you used is better.
I think that’s the output from mosquitto including the topic for information, and the actual received data is "99 %", so {{ value.split()[0]|int }} would be needed.