See alltheplaces.xyz for continuously updated straight-from-the-primary-source opening hours of chains of shops and restaurants, public facilities such as libraries, etc. This is probably as accurate as it gets AND you have the confidence of knowing exactly where the data came from (down to the URL) and when it was last checked.
Some OSM contributors go brand-by-brand/operator-by-operator in making sure OSM features have the most up-to-date opening hours added to them from matched ATP features. As such, OSM may be fairly accurate for chains too.
For a standalone shop or restaurant the opening hours situation is usually still better with Google Maps rather than OSM. There aren't enough OSM contributors who care enough to check and maintain opening hours for every shop, restaurant, fuel station, etc.
Both this and addresses is something that's really easy to survey with StreetComplete.
Google has the benefit of having their own street-level imagery for house numbers and street names, Android devices for real-time traffic info, and the ability to simply scrape web pages for shop data including opening hours. but in places with a reasonable number of active mappers, OSM is so much richer and more up to date.
At least in my experience for large retail chains or restaurant chains, hours on their websites are very accurate. I think where Google might benefit from calling to verify hours is, for example, where they've scraped https://schema.org/openingHours from the web page for an independent store, but the website has used an invalid/incorrect format, and some manual human check or clean-up is necessary, but this work would be too time consuming for each of 100,000's of independent stores which exist.
There are cases where a brand might have broken opening hours information such as "Mo- Tu - We 09 00 -4 p m". For alltheplaces.xyz scrapers, a human is often able to make sense of the situation and fix the problem through a few lines of extra code. Contacting the business to get the hours confirmed would only be necessary in very rare cases.
Where it would perhaps make the _most_ sense to contact a business to confirm opening hours is:
1. Ahead of public holidays to confirm planned temporary changes to opening hours. Many brands don't bother to update hours on their websites for such temporary changes, or at least don't list those hours until the week prior, which is usually too late for OSM, ATP, offline maps, etc which would ideally want to know these changes to hours up to a year in advance.
2. To confirm actual dates of permanent store/restaurant closures. Many brands in liquidation tend to fire the website developers (or can't pay them to work) and leave an outdated website online until the final shop or restaurant is closed. Commonly it's only some "CLOSING DOWN SALE LAST DAYS EXTENDED TO FRIDAY" post on social media that informs visitors accurately.
IME there is no reliable source: Stores' (including restaurants') own websites are inaccurate enough that I need to call them if I need to be sure. Yelp, etc. are at least equally inaccurate.
I'm sure it annoys the stores that keep their website and Yelp etc. updated but there is no way to know who is reliable.
Some OSM contributors go brand-by-brand/operator-by-operator in making sure OSM features have the most up-to-date opening hours added to them from matched ATP features. As such, OSM may be fairly accurate for chains too.
For a standalone shop or restaurant the opening hours situation is usually still better with Google Maps rather than OSM. There aren't enough OSM contributors who care enough to check and maintain opening hours for every shop, restaurant, fuel station, etc.