Your clients should know when they can expect a response and when you might be unavailable due to unforeseen circumstances.
If I’m out for three months, *someone* is doing each bit of my job in that time. Me coming back and wading through three months of emails where the majority of them will involve someone seeing the OOO and promptly emailing my cover instead, and trying to track down which ones did that and cc-ed me, which ones did that and *didn’t* cc me, and which ones fell off is just a terrible use of getting-back-up-to-speed time.

There’s a grim, apologetic vibe to these messages — I’m sorry I’m taking time for myself but I’ll try to check in on occasion! They’re a vivid reflection of a work culture that valorizes constant productivity and the near-total overlap of work and life. But they’re also do a terrible job of what they’re intended to do, e.g., set realistic expectations for both sender and recipient. A vague OOO message traps both parties in an uncomfortable liminal space where both productivity and rest go to die. The original sender is left unsure if they’ll be getting a timely response or a whether the email will go ignored for a time or forever. The original recipient has taken what is a rock solid excuse (time off) and cheapened it, offering a backdoor for email guilt to creep in.
I am out of the office from January 14 to 20, with only limited access to my emails and voicemail. Please be informed that this mail hasn’t been forwarded. I’ll come back to you as soon as possible.