This article can be summarized as such: consensus rules prevent certain TXs from being included in mined blocks. Mempool policy rate limits certain types of consensus-valid but otherwise undesirable TXs. They shape in a loose way how people use Bitcoin, but doesn't stop anyone who's determined enough to get some non-std TX mined.

Sharing a recent study of non-standard transactions that found their way into the blockchain during the last year and a half. Of the total 269 million transactions that the Bitcoin network confirmed in that timeframe only 20k violated standard mempool policy. That comes at about 0.0074% of the total, or one non-std transaction every 6 blocks on average.