Will seeing a Muggle death allow one to see Thestrals? Can anyone provide canon examples either way?
I think any death allows you to see a Thestral. I mean when Luna explains it, she never names you having to see a specific creature dying, wizard or non wizard.
You can see here. It is also in Harry Potter and the Order of the Phoenix
Harry Potter: [about the Thestrals] So why can't the others see them?
Luna Lovegood: The only people who can see them are those who've seen death. Harry Potter: So, you've known someone who died?
Luna Lovegood: My mum. She was quite an extraordinary witch, but she did like to experiment, and one day, one of her spells went badly wrong. I was nine.
Harry Potter: I'm sorry.
Luna Lovegood: Yes, it was rather horrible. I still feel very sad about it sometimes. But I've still got Dad.
All the people that saw them in the books saw wizards die, but that does not mean that it is a rule. You need to see death.
It probably has to be a human death.
As what the user Angew said it probably has to be a human death as well. Harry lived in a cupboard for a while and probably saw loads of bugs die. Also, Luna said people, so that seems self explanatory. Muggles and Wizards are both people.