On the GDPR issue, I don't believe it will be an issue when done properly, that being done in a secure way. One venue I frequented just had an open book at the entrance and a shared pen (plus sanitiser) - I did put correct details in that case but in hindsight if I came across similar again I may not do (or, of course, not frequent such a venue in the first place)
What makes you feel that someone taking details on an electronic device is more secure than a paper book?
I've just come back from having a few pints in my local. There were only a few of us in, but the bar staff pulled the folder out from under the counter, recorded my name and the time I walked in, and put it back beneath the bar. That's enough in my book, but I do live in a fairly small place. I'm not confident that a similar system would work in a pub that's busy.
Since pubs have reopened I only really go to two places. One, my local pub where they take my name, number and postcode on an ipad. Or rather, they take my number and postcode because being my local, they know my name. The other is a Conservative Club where they have an insecure book open on the bar to fill in yourself, it asks for your name and number. I can't say filling it in has particularly worried me, nor have I taken any notice of the names and numbers before mine.
And to get back to my point why it is a serious breach, mobile phones for the most part are far more than a line line handset. Even a moderately competent hacker would delight in having a list of names & mobile numbers freely available to them, even more so when those people might actually be in the same premises as them. But don't take my word for it, you can try it out for yourself by simply leaving a card with your name & current mobile number wherever you go, and wait to see how it will take before you start to get phising texts or calls, and maybe worse.
You mean a business card? Thousands of people have been leaving those with their name and number all over the place for years.
As I said, there is no arguing the point. I'm glad you agree. However your comparison with the telephone books of yesteryear and mobile phones is not accurate, which was my point and what you dodged.
The phone book used to have my full name, address and number in it, just like for millions of other people. It also, last time I had one at least, contained some people's mobile numbers. Why is that different?
Then there are all the people with their names and mobile numbers on their van, on the back of their sweatshirt, on their leaflets, business cards...