SLEP009 is all but accepted.

It proposes to make most parameters keyword-only.

We should do this by first:

• Merging ENH Add Deprecating Position Arguments Helper #13311
• Perhaps getting some stats on usage of positional arguments as per SLEP009: keyword only arguments enhancement_proposals#19 (comment)
• applying the deprecation to each subpackage. Checked means PR opened at least.
  • base
  • calibration
  • cluster
  • compose
  • covariance
  • cross_decomposition
  • datasets
  • decomposition
  • discriminant_analysis
  • dummy
  • ensemble
  • feature_extraction
  • feature_selection
  • gaussian_process
  • impute
  • inspection
  • isotonic
  • kernel_approximation
  • kernel_ridge
  • linear_model
  • manifold
  • metrics
  • metrics.pairwise
  • mixture
  • model_selection
  • multiclass
  • multioutput
  • naive_bayes
  • neighbors
  • neural_network
  • pipeline
  • preprocessing
  • random_projection
  • semi_supervised
  • svm
  • tree
  • utils

We might along the way establish rules of thumb and principles like "are the semantics reasonably clear when the argument is passed positionally?" As I noted on the mailing list, I think they are clear for PCA's components, for Pipeline's steps, and for GridSearchCV's estimator and parameter grid. Other parameters of those estimators seem more suitable for keyword-only. Trickier is whether n_components in TSNE should follow PCA in being positional... It's not as commonly set by users.