There's always been a "shortage of Advertisers" in my niche, so I contend with quite a lot of off-topic ads that I would intuitively block if I had sufficient on-topic inventory filling those spots.

Depends entirely on your niche and location.

For me, informational, UK, most popular hobby, there is no shortage of quality advertisers.

Any analysis of the amount and quality advertisers is hugely complicated. Much depends on the nature of your website (quality and uniqueness of content, presentation etc) as well as the niche you are in.

Occasionally exterior forces (outside the web) will have an effect on advertising spend, changes in market forces, economic reports, stock market, etc. can intrude. Fortunately these are generally short term and merely part of of the business cycle. One becomes concerned if some changes last more than 90 days, or remain downward without fluctuations.

It does seem that a few (large) advertisers have adjusted their ad spends on digital platforms over the last year, or changed their focus on where those digital ads are placed (think social media).

There is a school of thought - especially among respected online news outlets who themselves are affected - that Facebook and Google Search itself is swallowing up advertising budgets.

Who Knows? Only those people who have the day job of knowing the true facts.

Meanwhile as usual we speculate among ourselves.

The presence of spammy ads is a direct indication of a lack of demand for you website, niche and/or audience. The spammers are not out there paying premiums to target a specific audience. They are bottom fishing, low balling. The ad-balancer proved this. With the ad-balancer active all these ads were eliminated from my site.

@IanCP

Facebook and Google Search itself is swallowing up advertising budgets.

I was looking into starting an Adwords campaign (new account) for a new site, there was an option to include display networks in the campaign, but there was no option to remove search ads. So you could have search and display, or only search. It might have I missed something, but it certainly wasn't made easy for a first time user. This would support the claim.

So you could have search and display, or only search.

When it first came into being back in 2003, the advertising was on search [I think] and only those AdSense invited who had huge page views per day as publishers.

We - the great unwashed masses were only allowed in to join in June, 2003. Initially I was sceptical but after a few days I finally put the code into my SSI page [for site wide above the fold] in place of Amazon. The first full AdSense day alone beat a whole month of Amazon.income. The rest is history of course. Now for me AdSense is competing with Amazon, and may be relegated to a secondary role at the end of this month. A 50/50 decision.

[ADDED] I still keep wondering whether small publishers are of any real value to Google at all nowadays. Collectively we were once "The Internet", no big names to speak of. Then again I've being saying that about Amazon and their programme for nearly twenty years now - we're still here, sort of.

Is there a shortage of Advertisers at the moment?

Nope.