UNESCO provide this broad, and detailed, summary of how different countries across the world have reacted to Covid in terms of the end of school/pre-university examinations.

This blog entry from the World Bank also asks: "High-stakes school exams . . what is the best approach?"
The situations may, however, have changed since these articles were published: on the 4th and 11^th April 2020 respectively.
They still provides a nice list of the range of possible options being considered in different countries and continents.
Please share what is happening in your country’s national exam’s system in the comments below.

US: SATs (May and June 2020) cancelled.
For the August, September, October, November and December 2020 dates (all of which should be available to international students as well, except for November) whilst this regularly update page from Princeton confirms they will run, "no matter what" quoting the college board: “In the unlikely event that schools do not reopen this fall, College Board will provide a digital SAT for home use, like how we’re delivering digital exams to 3 million AP students this spring.” Other news, such as this Forbes magazine article, suggests that places are limited and many centres not open: "54% of locations have said they will open and some of those are limiting capacity". The article links to the college boards "test centre closing" page.

This article provides a good overview of the AP online exams process, which went ahead (but online) in May 2020. College board's AP 2020 exams run online page.

Canada: exams cancelled, results based on student marks pre-Covid.(Ottawa)

China(Hong Kong) and India
China chose to postpone to July 7th and 8th the Gaokao university entrance exams for 10 million students

Hong Kong chose the “postpone, but still run, exams” option too.

India either had their exams, as usual, pre-covid in Feb/March (?), so weren’t affected, or postponed them to July.

Europe

This Telegraph article seems to provide a good summary of a number of European countries handling of school leaving certificate grade award.

It looks like Spain and Germany (from the countries discussed in the article) were the only ones to run examinations (if, for Spain, in reduced format)(?).

Inthinking’s Biology author, David Faure, posted this decision from Norway where "course participation counts for 80 percent" in the Norwegian high school leaving certificate:

The "postpone/take exams" would probably be my preferred model, be it online, postponed to a later date or other format. In the case of the UK and US, would anglophone (or EU?) unis have been prepared to adapt all their entrance systems to wait until later to start planning the reception of students in 2020? Maybe the bad publicity around all the 'algorithm' approaches was a (regrettably) unavoidable 1st attempt to make such an approach more acceptable in the future (for govts, schools, votres, unis, students?)?

The ‘exam’ format seems reasonably 'fair' i.e. 'same conditions for everyone' and 'outcome of their own efforts' (rather than 'historical data' approaches), in a very difficult situation, for a national exam. Students within one nation have all had the same Covid schooling restrictions and start & end dates of Covid19 interruptions etc.

For an international exam board, each country’s very different start and end points for the arrival of Covid on their territory, different government school closure policies, online learning, access to technology etc. makes it less clear what the ‘fair’ way forward would be.

I imagine that the majority of IB students (over 50%, maybe over 70%?) go on to Anglophone (US, Canadian, UK, Australian, NZ, South African etc) universities.
The IB, may be more influenced by these country's policies than others if this is the case(?).

2020 to 2021 Academic year
It will be interesting to see how different countries, and the IB, respond to events during 2020/21 and if any kind of general, cross country, agreement on the “best” approaches are reached, or if, given different historical and cultural contexts, approaches remain diverse.