What is the clear evidence that secretions are an event that invalidates ablution, and is it true that Ibn Uthaymeen ruled that they do not invalidate it in the latter part of his life?

Scholars have agreed that minor ritual impurity (hadath) nullifies ablution (wudu'). The impurities upon which there is consensus are: urine, feces, pre-seminal fluid (madhi), prostatic fluid (wadi), loss of consciousness, and deep sleep. They differed on other matters such as kissing, touching the penis, nosebleed, and blood from cupping.

The correct view is that moisture from a woman's vagina and anything else that exits from the two private parts nullifies ablution, by analogy with the explicitly mentioned impurities. As for the hardship of performing ablution for every prayer, it is a bearable hardship, and a woman can combine two prayers with a single ablution.

Sheikh Ibn 'Uthaymeen issued a fatwa stating that continuous discharge from a woman is pure and does not necessitate ablution for every prayer unless ablution is nullified by another impurity, while preferring ablution for each prayer if it is easy to do so.