This prerogative of the Crown remains in existence. As an example, the Governors-General of various Commonwealth countries, such as Australia, also feature this quirk of law, because each Governor-General acts on behalf of the Crown, and thus--legally, literally, per the letter of the law in those countries--retains the right to absolute veto. A Governor-General using this power would be somewhat less scandalous, but still would very likely have negative consequences.
Same thing goes for Parliament itself with its "absolute sovereignty" concept. Parliament's sovereignty in the UK is truly absolute. It can write whatever laws it wants, even laws that are impossible to enforce or impossible to comply with. If Parliament wished to, it could abolish elections tomorrow and rule as an oligarchy. If Parliament wished to, it could declare that all persons must wear pants with legs that are shorter than one foot long and longer than two feet long, even though it is logically (and physically) impossible for any pair of pants to meet that requirement. Nothing--not even Parliament itself--can limit the scope of Parliament's legislation, and UK courts functionally cannot declare any Parliamentary law unconstitutional. (Note that I did not say literally cannot; there are contexts where it is possible, but they're exceedingly narrow.) Per the rulings of Lord James Reid in 1965:
"It is often said that it would be unconstitutional for the United Kingdom Parliament to do certain things, meaning that the moral, political and other reasons against doing them are so strong that most people would regard it as highly improper if Parliament did these things. But that does not mean that it is beyond the power of Parliament to do such things. If Parliament chose to do any of them, the courts would not hold the Act of Parliament invalid."
Most of this is not very accurate.
On the possibility of legislation being found to be unconstitutional by the Supreme Court of the UK, consider this blog by Adam Tucker:
Adam Tucker: The Rwanda Policy, Legal Fiction(s), and Parliament’s Legislative Authority
One doesn't have to go all the way with common law constitutionalists to reach conclusions about the scope of judicial review in the UK that are different from what you state in your post.
The Parliament of the UK also clearly can limit its own legislative authority, at least as far as implied repeal is concerned. This is illustrated by the European Communities Act and
Factortame - Jeffrey Goldsworthy has what is probably the best account of how this possibility is to be reconciled with traditional accounts of parliamentary sovereignty.
As to the situation in Australia, the legal character of the Governor-General's obligation to act on advice - except when exercising reserve powers - is a matter of dispute. The leading textbook, which is
Zine and Stellios's The High Court and the Constitution, defends the view that at least some of these matters which historically been taken to be matters of convention are really matters of law. (That is not the same thing as saying that they are justiciable, although even then I think there may be reasonable arguments either way as to whether, eg, the High Court of Australia would issue an injunction to restrain the Governor-General appointing a minority leader Prime Minister after an election.)
And when you say that a unilateral veto of legislation by the Governor-General would be "slightly less scandalous", that is utterly wrong. In fact it would precipitate a constitutional crisis, although one that would likely be easily resolved: the Prime Minister would advise the King to dismiss the Governor-General, and then whoever those powers were then vested in (a new Governor-General, or an acting Governor-General) would assent to the legislation at issue.
In this context, it's also worth noting that in Australia the "Medevac" legislation that was enacted despite government opposition, because the government temporarily lost control of the House of Representatives, was assented to. In the UK, there was debate around whether the government might advise Her Majesty not to assent to legislation that had been enacted in similar circumstances (ie when the government had lost control of the House of Commons), but the government did not end up precipitating that particular constitutional crisis (Brexit having already given rise to sufficiently many of those). So describing the principles around Royal Assent as a "gentelman's agreement" is completely wrong, not to mention quite misleading.