bash script to delete folders with a name_date older than 90 days

The key is to convert the date folder names into Unix Epoch time so you can easily compare them.

#!/bin/bash

dataDir="/abs/path/to/data"
recentDir="/abs/path/to/data_recent"
daysToKeep=90
minKeepEpoch=$(date --date "$daysToKeep days ago" +%s)

# Create new links for folders that are within $daysToKeep
while IFS= read -r -d $'\0' dir; do
  dirName=${dir##*/}
  dirEpoch=$(date --date ${dirName//./} +%s)
  (( dirEpoch >= minKeepEpoch )) && ln -s -t "$recentDir" "$dir"
done < <(find "$dataDir" -mindepth 1 -maxdepth 1 -type d -print0)

# Remove links that are older than $daysToKeep    
while IFS= read -r -d $'\0' link; do
  linkName=${link##*/}
  linkEpoch=$(date --date ${linkName//./} +%s)
  (( linkEpoch < minKeepEpoch )) && rm "$link"
done < <(find "$recentDir" -mindepth 1 -maxdepth 1 -type l -print0)

Proof of Concept

Note that ./data_recent was pre-populated with an outdated link that will be removed

$ tree ./data
./data
├── 2010.01.01
│   ├── f1
│   └── f2
├── 2020.02.27
│   ├── f1
│   └── f2
├── 2020.02.28
│   ├── f1
│   └── f2
├── 2020.05.27
└── 2020.05.28
    └── f1

12 directories, 0 files

$ tree ./data_recent/
./data_recent/
└── 2010.01.01 -> /abs/path/to/data/2010.01.01

1 directory, 0 files

$ ./syncFolders.sh
$ tree ./data_recent/
./data_recent/
├── 2020.05.27 -> /abs/path/to/data/2020.05.27
└── 2020.05.28 -> /abs/path/to/data/2020.05.28

2 directories, 0 files