passwordbreachd iOS 14.4 have my iPhone been hacked?

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware if it is updated to the latest iOS/iPadOS unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed on your device or/and have Jail Broken. 

It (Hacking) also depends on how careful are you in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password etc with your friends and family members.

Be careful when sharing the device's sensitive and valuable information with friends and family members.

Thumb Rules:

1. Enable  Guided Access before handing over an iPhone to kids
2. Don't share Apple IDs
3. Don't Jail Break
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing

passwordbreachd is a Daemon for Safari AFAIK. Its possible that the process had a memory leak or something that caused it to slow down your device or exceed JetSam memory limits and crash. As long as this doesn't occur on a regular basis, its nothing to worry about. I'm not sure what passwordbreachd actually does, but it is an official process and nothing to worry about. Just curious- did you happen to be signing in to something using a "sign in with apple" option when this happened? Sometimes I have issues when doing that in Safari, and I get the same issue in my device log.

As you say, you're not an expert on device logs. Me neither - I never look at them. They mean little to most people if they are not developers. Best follow the advice given in the post by Sravankra and be on the lookout for any day-to-day signs that your phone or Apple account has been compromised, rather than trying to read anything into those logs.