There’s nothing awful or offensive about this message, but it’s also not very good. Yes, it provides the courtesy of letting the sender nominally know that you’re going to be slower than usual to respond. That’s nice. The problem is in this bit: “may be slow to respond to email.” Another popular variation: “might be slower than usual to respond.”
Vacation Tracker helped me book some days off, so you will not be able to reach me until *date*! I will be busy surfing the waves in Portugal/ climbing Kilimanjaro/ taking pictures on the Great Wall of China/ exploring wildlife in Tanzania.

I wrote the above comment off the top of my head. I wish I had time to rewrite and edit it. I would have changed “their goldfish” to “a spider they accidentally stepped on”, and would have added more detail to the story of the sister’s death (e.g. “her Pomeranian yapping” rather than the less descriptive “her dog barking”). Unfortunately, I could not do the thorough writing job required for that comment because someone close to me recently … – The person whose out of office advertised his gig on the weekend, for anyone in travelling to [city] – The people in a certain department who have taken to saying things like “if you really need to contact me, call 000-YYY-XXXX where Y is the square root of [insert numbers] and X is the year plutonium was discovered.” – The ones where people have an auto response saying they only check their emails once a day between 1-2pm – “I’m on research leave and I may be slow to reply.” (Whereby it is guaranteed they will reply immediately, because academics do not *really* take breaks).
If people know you are on vacation or maternity leave, for instance, they are more likely to wait for your return to follow up. If you are at a work conference, however, they might still expect you to check your email or provide updates.