Definition: The modified broad match was a type of match used in Google Ads somewhere between broad and phrase match.
It was distinguished in that it provided more control than broad match and more freedom than phrase match.
It disappeared as an option in July 2021.
It worked by adding a “+” sign in front of one or more words in a phrase composed of keywords, when the selected match type is broad.
Characteristics of the modified broad concordance
In order to match the keyword, the words with the ‘+ ‘ sign in front of them had to be present in the search.
The order of the keywords did not matter.
Unlike broad match, modified broad match did not show an ad for synonyms or related searches, so it gave a higher level of control.
Also, unlike phrase matching, the order of the words was not important which gave it more flexibility.
Example of modified broad concordance
For example, if we use as keyword +clothing +shop, the ad will be activated with the searches “clothing store”, “clothing store in Seville” and “cheap clothing in store”. However, it will not be activated with “clothing store”, because the word store is not found in the search. In the modified broad match, as in the broad match, it is very important to use negative keywords to narrow down the searches in which the ad jumps.
Modified broad match discontinuity in Google Ads
Since February 2021, the modified broad concordance has been replaced by the phrase concordance. Match types are simplified by combining the phrase match control with the extended coverage of the broad match modifier, which is no longer available. 
Source: https://support.google.com/google-ads/answer/10286719?hl=es