Why is there no information about the standard of the property (standard of decoration, luxury equipment, garden area, ...), again which would be necessary for a landlord to make an informed decision.
Perhaps consider balancing that by replacing "landlord" with "landlord and tenant".
Their reason, should they answer, would probably be - that they have no idea what state of the properties on the register are in. There is a straw to grasp in the BER rating which might indicate a property is quite new.
They know nothing about how old, cheap, nasty and moldy bathrooms are, how smelly & grubby the carpets, how basic the kitchen, how depressing or uplifting the experience of standing in the property. They know nothing about the quality - not even if there's parking - and to suggest these things are not relevant to establishing a fair market rent is absurd.
There is one group of landlords who will be pleased with these new rent rules and the pressure to price in line with the RTB rent register - slumlords. They can push up the rents on their kips to be at or just less than the highest of the RTB's selection of 10 and the RTB's IT system will never flag them as charging more than a fair price, and any complaint by a tenant will be dismissed since the slumlord is being fair by the RTB's own definition.
How long the property has been under RPZ rules; when did the current tenancy start (+ anything else you can think of)
I think "when did the tenancy start" is indicated within the RT number, no? But is in any case a bit of a distraction - any new tenancy of a property previously under RPZ rules is no more relevant than an old one because the same restrictions on rent level have applied. It's important that the focus is shifted on to what matters, which is the first part of your sentence.
"How long the property has been under RPZ rules?" - or asked a different way
"when was the property first introduced the the rental market, or reintroduced after an absence of more than two years?"
...as these would indicate a property for which the rent may reflect recent actual market rent.
They would probably obfuscate in a reply to that with something vague about only being able to look at data - so a data sympathetic translation for them might be something along the lines of
"the latest date of when each property first appeared on the register, or reappeared after an absence of two years or more"
Their data will be a bit unreliable especially going back several years (eg landlord has property become vacant, leaves vacant for two years perhaps while renovating, but forgot to, or was unable to, end the registration of the previous tenancy) but even with some errors that would be a vast improvement in the relevancy of the data used by landlords and tenants to 'discover market price' - and it is in any case the more recent, cleaner (because of annual re-registration), data that is most relevant.