https://www.kdnuggets.com/2020/07/r-squared-predictive-capacity-statistical-adequacy.html
R squared Does Not Measure Predictive Capacity or Statistical Adequacy
The fact that R-squared shouldn’t be used for deciding if you have an adequate model is counter-intuitive and is rarely explained clearly. This demonstration overviews how R-squared goodness-of-fit works in regression analysis and correlations, while showing why it is not a measure of statistical adequacy, so should not suggest anything about future predictive performance.
By Georgi Georgiev, Webfocus.
“The R-squared Goodness-of-Fit measure is one of the most widely available statistics accompanying the output of regression analysis in statistical software. Perhaps partially due to its widespread availability, it is also one of the most often misunderstood ones.”