A missing domain has two causes. The publisher removed the link, or our monthly crawl has not caught it yet.
So one absence is a prompt, not a finding. Two absent months in a row, plus a visual check of the page, is a finding you can show the vendor.
If you want real-time alerts, dedicated monitoring tools exist, and they will beat a monthly snapshot on speed. What this routine buys is a trend line for a few cents, and a dated record when you ask for a replacement.
Choosing what to record
Keep the columns few. Domain, URL, price, date bought, and one cell per month. More columns invite more work that nobody does.
Add a note field for events you know about. A site migration on the publisher's side or a CMS change often explains a mass removal, and the note saves an argument next quarter.
If you bought many placements, summarise with a rate: removed divided by bought, with the count of placements stated. Ten placements and two removals is a rate of 20 percent, but n is 10, so quote the n.
It cannot tell you why a link vanished, whether the page was deleted, or whether Google ever counted it. It also cannot warn you in real time; for that you need a monitoring service, and our monthly data is the wrong tool.
What it gives you is a tidy history, a rate, and a conversation with a vendor that begins with evidence. Google introduced rel=sponsored and rel=ugc in September 2019, and many paid placements now carry such attributes, so check the markup when you inspect a page: a sponsored link may be present yet valued differently.
Why bother with a sheet
Placements are bought on trust. A publisher's note that the link will stay up is a promise, and promises about links tend to last as long as the page does.
With a dated sheet you can say 'present in March, April and May, absent in June and July' and ask for a replacement. Without one, you are offering a feeling. A feeling is easy to refuse.
The cost is trivial: at 2 credits per pull, twelve monthly pulls are 24 credits, or $0.0024.
A hypothetical sheet after four months
Invented placements. P means present in that month's pull, dash means absent.
Placement
Month 1
Month 2
Month 3
Month 4
blog-a.example
P
P
P
P
blog-b.example
P
P
-
-
news-c.example
-
P
P
-
What the sheet says
The second row is the one to chase: two absent months and a check by hand. The third row is odd, and looks like a crawl gap in month one rather than a removal.
Count your own loss rate as removed placements divided by bought, with n stated, and keep it for the next purchase.