I’m trying to find out what is holding back DataStructures package.
Installed version is v0.17.20, latest version is 0.18.4.
Attempting to explicitly add the latest version of DataStructures  reveals a seemingly weird incompatibility with ReverseDiffSparse, JuMP and Compat.
It seems ReverseDiffSparse is used by older versions of JuMP, but is not a dependency of the current and installed version (JuMP 0.21.3), and ReverseDiffSparse does not appear in my Project.toml or Manifest.toml files.
(v1.5) pkg> add DataStructures@0.18.4
  Resolving package versions...
ERROR: Unsatisfiable requirements detected for package ReverseDiffSparse [89212889]:
 ReverseDiffSparse [89212889] log:
 ├─possible versions are: 0.8.2-0.8.6 or uninstalled
 ├─restricted by compatibility requirements with Compat [34da2185] to versions: uninstalled
 │ └─Compat [34da2185] log:
 │   ├─possible versions are: [1.0.0-1.0.1, 1.1.0, 1.2.0, 1.3.0, 1.4.0, 1.5.0-1.5.1, 2.0.0, 2.1.0, 2.2.0-2.2.1, 3.0.0, 3.1.0, 3.2.0, 3.3.0-3.3.1, 3.4.0, 3.5.0, 3.6.0, 3.7.0, 3.8.0, 3.9.0-3.9.1, 3.10.0, 3.11.0, 3.12.0, 3.13.0, 3.14.0, 3.15.0] or uninstalled
 │   └─restricted by compatibility requirements with DataStructures [864edb3b] to versions: [3.0.0, 3.1.0, 3.2.0, 3.3.0-3.3.1, 3.4.0, 3.5.0, 3.6.0, 3.7.0, 3.8.0, 3.9.0-3.9.1, 3.10.0, 3.11.0, 3.12.0, 3.13.0, 3.14.0, 3.15.0]
 │     └─DataStructures [864edb3b] log:
 │       ├─possible versions are: [0.9.0, 0.10.0, 0.11.0-0.11.1, 0.12.0, 0.13.0, 0.14.0-0.14.1, 0.15.0, 0.16.1, 0.17.0-0.17.20, 0.18.0-0.18.4] or uninstalled
 │       └─restricted to versions 0.18.4 by an explicit requirement, leaving only versions 0.18.4
 └─restricted by compatibility requirements with JuMP [4076af6c] to versions: 0.8.2-0.8.6 — no versions left
   └─JuMP [4076af6c] log:
     ├─possible versions are: [0.18.3-0.18.6, 0.19.0-0.19.2, 0.20.0-0.20.1, 0.21.0-0.21.3] or uninstalled
     ├─restricted to versions * by an explicit requirement, leaving only versions [0.18.3-0.18.6, 0.19.0-0.19.2, 0.20.0-0.20.1, 0.21.0-0.21.3]
     ├─restricted by compatibility requirements with DataStructures [864edb3b] to versions: 0.18.3-0.18.6 or uninstalled, leaving only versions: 0.18.3-0.18.6
     │ └─DataStructures [864edb3b] log: see above
     └─restricted by compatibility requirements with Compat [34da2185] to versions: [0.18.6, 0.19.0-0.19.2, 0.20.0-0.20.1, 0.21.0-0.21.3] or uninstalled, leaving only versions: 0.18.6
       └─Compat [34da2185] log: see above